The present invention generally relates to a magnetic recording and/or reproducing apparatus, and in particular to a magnetic recording and/or reproducing apparatus in which a digital audio signal which has been subjected to an offset-multi-phase differential phase shift keying modulation such as an offset-quadrature shift keying modulation, an offset-quadrature differential phase shift keying modulation, or a differential phase shift keying modulation such as a quadrature differential phase shift keying modulation is recorded in and/or reproduced from a deep layer portion of a magnetic layer of a magnetic tape, and a video signal is recorded in and/or reproduced from a surface layer portion of the magnetic layer. Furthermore, the present invention relates to a magnetic recording and/or reproducing apparatus in which a frequency modulated audio signal is recorded in and/or reproduced from the deep layer portion of the magnetic tape together with the modulated digital audio signal.
A magnetic recording and/or reproducing apparatus has been proposed in which a digital audio signal which has been modulated by a multi-phase differential phase shift keying is recorded in and/or reproduced from the deep layer portion of the magnetic tape, and the video signal is recorded in and/or reproduced from the surface layer portion of the magnetic tape. In the apparatus of this type, an input color video signal in conformance with a standard color system such as the NTSC standard system is converted into a frequency division multiplexed video signal in which a frequency converted carrier chrominance signal and a frequency modulated luminance signal are frequency division multiplexed. The carrier chrominance signal has been converted into a frequency range lower than that of the frequency modulated luminance signal. The frequency division multiplexed video signal is passed through a recording amplifier and a switching circuit, and is supplied to rotary video heads. The rotary video heads record the frequency division multiplexed video signal in a surface layer portion of a magnetic tape and form video tracks.
On the other hand, input right and left channel analog audio signals are respectively converted into pulse code modulated (PCM) signals and are subjected to a time division multiplexing process. A time division multiplexed PCM signal is modulated in accordance with the offset-quadrature phase shift keying (hereafter referred to as OQPSK) modulation. An OQPSK-modulated digital audio signal is passed through a recording amplifier and a switching circuit and supplied to rotary audio heads. The rotary audio heads record the digital signal in a deep layer portion of the magnetic layer of the magnetic tape and form audio tracks.
During a reproducing operation, the frequency division multiplexed video signals read out from the video tracks by the rotary video heads are passed through the switching circuit and a video signal processing circuit. The processing circuit reproduces the original color video signal from the read-out video signal.
On the other hand, the digital audio signals read out from the audio tracks are passed through a switching circuit, a reproducing amplifier, an equalizer and an OQPSK demodulator, and supplied to a digital signal processing circuit where the reproduced original PCM signal is obtained. Thereafter, the reproduced PCM signal is converted into the original analog audio signal by a digital-to-analog converter, which simultaneously outputs the reproduced right and left channel audio signals.
As described above, in the conventional recording and/or reproducing apparatus having the rotary audio heads, the OQPSK modulated digital audio signal is directly recorded on the magnetic layer of the magnetic tape. This type of the recording and/or reproducing apparatus has been disclosed in the following document: Arai, et al, "A STUDY ON THE DIGITALIZATION OF AUDIO SIGNALS FOR VIDEO TAPE RECORDER", International Conference on Acoustics, Speech and Signal Processing, pp 29-33, 1986).
However, it should be noted that in the above magnetic recording and/or reproducing apparatus, the OQPSK modulated digital audio signal is directly recorded on the magnetic tape which has the non-linear transmission properties. Hence, a cross modulation distortion may occur. This causes a noise spectrum in a frequency range of the frequency converted carrier chrominance signal. It should be noted that no noise spectrum exists at the time of the recording.
For this reason, the noise spectrum included in the reproduced signal during the reproduction of the video signal by the rotary video heads degrades the color signal-to-noise ratio. Further, the distortion included in the reproduced audio signal because of the non-linearity of the magnetic tape during the reproduction of the audio signal by the rotary audio heads degrades the error rate of the reproduced digital audio signal.
Moreover, in order to record and reproduce the OQPSK-modulated digital audio signal on and from the magnetic tape, it is necessary to record and reproduce a reference phase signal at the same time when the OQPSK modulated digital audio signal is recorded and reproduced.